1. Field of the Invention
The present invention relates in general to apparatus for electrical switching between arbitrary numbers of input and output terminals, and in particular to the field of telephone headset adapters and the wiring configuration switching for compatibility with the various handset port interfaces.
2. Background of the Invention
In order to interface properly a telephone to a headset, a telephone headset adapter must correctly match the transmit and receive lines of the handset to the transmit and receive lines of the headset. To obtain broad product acceptance, the telephone headset adapter must be compatible with a large variety of telephones. However, a major problem in providing a telephone headset adapter that is compatible with most telephones is the lack of industry standards for handset port wiring. Most telephones use a four pin modular connector to plug in the handset. However, pin assignment for these four pins is not standardized. In addition, some manufacturers use a three wire interface with the fourth pin providing power for handset electronic systems.
Traditionally the connection between the headset and handset has often been achieved using either multi-pole bit switches or combinations of readily available, or custom built, slide switches. These solutions thus require the use of "codes" or switch setting combinations which are non-intuitive to the end user. The switches are typically small and difficult to manipulate with one's fingers, thus making it difficult for the user to configure correctly the telephone headset adapter. Furthermore, if the "code" is lost, misplaced, or unknown, the user has little recourse but to try different switch position combinations, cycling systematically through the different possibilities. For the average user this might be difficult to do. Clearly it would be advantageous for the user to be able to cycle quickly and easily through a range of possible combinations or configurations.
Another drawback is that most of the traditional switch solutions are only dedicated to a subset of possible combinations of handset and headset connections. As a result, when a new telephone wiring combination is needed for a new telephone a complete product redesign of the telephone headset adapter is required to adapt to the new wiring configuration.
Other traditional solutions include a silicon "crosspoint" switch chip that can connect any one of (typically) four input lines with any one of (typically) four output lines. This solution suffers from the drawback that extensive protection circuitry is required to prevent damage to the crosspoint switch from line voltages, RF interference and ESD events. Furthermore, if isolation between the telephone and the adapter system is required, power from the adapter system must be provided to the isolated telephone interface section of the circuit. This requires extra cost. Clearly it would be more cost effective to have an adapter system which is isolated from the telephone and which has a passive telephone interface section.